1. Field of the Invention
The present invention relates to a two-unit zoom lens system, an exchangeable lens using this two-unit zoom lens system, and further an electronic image taking apparatus using this two-unit zoom lens system.
2. Description of the Related Art
There is expanded a market of a single lens reflex (SLR) camera using an electronic image pickup device such as a CCD image sensor or a CMOS type image sensor. Heretofore, a conventional small-sized zoom lens system including the standard angle of field with respect to the SLR camera for a silver salt film is constituted of a first lens unit having a negative refractive power and a second lens unit having a positive refractive power.
Moreover, as a two-unit zoom lens system of a negative-positive type having a long back focal length (the optical path length between the rearmost surface of the lens system and the image plane), there are proposed several zoom lens systems usable with a small-sized image pickup device. As the two-unit zoom lens system of the negative-positive type in which the number of lenses constituting the zoom lens system is small, there are known systems disclosed in Japanese Patent Application Laid-Open Nos. 2001-215407, 2000-187157, 2001-21806 and 1-183615.
In the two-unit zoom lens system described in Japanese Patent Application Laid-Open No. 2001-215407, an aperture stop is disposed on the image side of the second lens unit. There is also disclosed a two-unit zoom lens system in which an off-axial chief ray is considerably inclined with respect to the image plane on the exit side of the zoom lens system.
Moreover, in the two-unit zoom lens systems described in Japanese Patent Application Laid-Open Nos. 2000-187157, 2001-21806 and 1-183615, the whole length is long as compared with a size of the image pickup device. When the system is used with a large image pickup device, the whole length increases. A magnification changing ratio is small.
As compared with the SLR camera for the silver salt film, in the SLR camera in which an electronic image pickup device such as a CCD image sensor is used, in general, a diagonal length of an image pickup surface is small. To prevent so-called shading phenomenon, from the center of the image pickup surface to a periphery thereof, a light flux needs to be incident on the image pickup surface in a state nearly vertical state with respect to the image pickup surface. A space for disposing a low pass filter and the like is required. Therefore, it is necessary to increase a ratio of a back focal length fb of a photographing lens with respect to the diagonal length. When this ratio is increased, an additional function can be introduced. On the other hand, if a large back focal length fb is required, such a lens constitution is required that the large back focal length fb can be secured on a wide-angle end of a region of the magnification change.